At present, a prepolymer (hereinafter referred to as a TDI curing agent) of virulent toluene diisocynate (TDI) and trimethylolpropane (TMP) is mainly used as a curing agent for double component polyurethane coatings, adhesive agents and printing ink. People have paid more and more attention on the environmental pollution problem caused by residual monomers of the TDI curing agent. Overseas manufacturers mostly adopt a rotary thin film evaporation technology to reduce the content of free TDI monomers; however, domestic enterprises do not handle the free TDI monomers due to the complex process engineering and expensive devices; therefore, the content of the free TDI monomers is generally larger than 2% in average, and even as high as 10% sometimes. In recent years, a number of works have been done on developing an environment friendly MDI curing agent to substitute the TDI curing agent in China. Some invention patents have been disclosed and some MDI curing agent products have been launched in the market. However, some problems to be resolved still exist in the process of replacing the TDI curing agent for use, wherein the prominent problems are that the NCO percentage of the MDI curing agent is lower than that of the TDI curing agent and the curing speed is faster, which are not suitable for application in some specific conditions, and obstruct the spreading and popularization of the MDI curing agent.
China patents CN101230124B and CN101440149B disclose a method to prepare an MDI prepolymer curing agent having high solid content and low viscosity by using dibasic alcohol to modify 4,4′-MDI firstly, and then solely or be mixed with an isomer 4,2′-MDI in any weight ratio, and react with trimethylolpropane. Although the NCO percentage of the curing agent can be improved and the curing speed can be reduced through selecting dibasic alcohol having different molecular weights and structures, the regulating degree is limited and the problems of low NCO percentage (9˜15%), and faster solidification rate still exist, which affect some application ranges and construction pot life.
The MDI prepolymer curing agent prepared by China patent CN1357558A by adopting trimethylolpropane, castor oil and 4,4′-MDI and the MDI prepolymer curing agent prepared by China patent CN1357559A by adopting trimethylolpropane, 4,2′-and 4,4′-MDI both have the technical defects of low solid content of a curing agent system, lower NCO percentage (10˜15%) and higher content of free isocyanate monomers in the system, which are easy to cause fast solidification in the prophase of construction and slow solidification in the anaphasis of construction.
Generally, the NCO percentages of the TDI prepolymer curing agents sold in the market are 16˜48% (as high as 19% theoretically, because a polymer consumes partial NCO), the NCO percentages of the MDI prepolymer curing agents of the foregoing several patents are all lower than the number. Low NCO percentages of the curing agent will increase the comprehensive cost of the user, bringing difficulty spreading and applying the MDI curing agent. Analytically, the reason why the NCO percentage of the MDI prepolymer curing agent is lower than that of the TDI prepolymer curing agent is that the molecular weight of the MDI is larger than that of the molecular weight, resulting in that the NCO content of unit quantity is reduced after the MDI is prepolymerized with same trimethylolpropane. Therefore, the polybasic alcohol molecular weight of the prepolymer shall be reduced as much as possible so as to improve the percentage of the MDI curing agent; thus, the inventor gets an idea of replacing the polybasic alcohol by water to react with the MDI for trying to prepare MDI biuret triisocyanate.
Since 1958, people have just known two preparation methods of aliphatic isocyanate having a biuret structure. The first one is the so-called water method, wherein aliphatic diisocyanate is reacted with water to form urea and subsequently form biuret, and HDI biuret has been produced industrially. The second method is the so-called aliphatic diisocyanate/aliphatic diamine method, wherein urea is directly prepared through isocyanate and amine, and then a biuret reaction is performed. A “Method for Preparing Polyisocyanates Having Biuret Structure” disclosed by China patent CN1188445C applied by Germany Bayer is just a method of continuously preparing the polyisocyanates having a biuret structure by using aliphatic and/or cycloaliphatic diisocyanate and aliphatic and/or diisocyanate diamine. So far, the polyisocyanates having a biuret structure are all prepared by using aliphatic diisocyanate, and there is no report about using aromatic diisocyanate to prepare the biuret polyisocyanates.
Since an induction effect exists between two —NCO groups on a benzene ring, the activity of the aromatic diisocyanate are higher than that of the aliphatic diisocyanate, and the manufacturing difficulty is enlarged, which is a technical problem to be solved by the international persons in polyurethane industry.
The present invention, directing at preparing an MDI biuret structure triisocyanate with water, proposes a preparation method of a modified aromatic biuret, which preferably solves the problems of the existing MDI prepolymer curing agents.